The invention relates to improvements in and relating to cathodes for use in the electrolysis of an aqua-solution of alkali halide.
In the process for the electrolysis of an aqua-solution of alkali halide such as NaCl or KCl to obtain the corresponding caustic alkali and chlorine, the aqua-solution is supplied naturally to an electrolytic vessel provided therein with at least an anode and at least a cathode. When electrolytic voltage is applied between the cathode and the anode, the caustic alkali will develop in the electrolyte contacting with the cathode, while gaseous chlorine and gaseous hydrogen develop at the anode and the cathode, respectively, when expressed in a generalized conception.
In the diaphragm electrolysis, DSA-electrodes are commonly used as the anodes, while steel is generally used as the material of the cathodes, according to the presently employed technique.
When expressing generally the whole progress of the electrolysis, the anodic reaction progresses in the following sense: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.- (a)
and the cathodic reaction progresses in the following sense: EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- (b)
More precisely, the cathodic reaction is called to take place in the following way: EQU H.sub.2 O+e.sup.- .fwdarw.H (adsorption)+OH.sup.- (c)
In this way, monoatomic hydrogen is adsorbed on the cathodic electrode surface. In the alkaline medium, the once adsorbed hydrogen is desorbed in the following one step or two step process: EQU 2H (adsorption)+H.sub.2 O.fwdarw.H.sub.2 ; or (d) EQU H (adsorption)+H.sub.2 O+e.sup.- .fwdarw.H.sub.2 +OH.sup.- (e)
The above hydrogen desorbing stage (d) or (e) is said to be decisive of the hydrogen overpotential.
On the other hand, the hydrogen-generating potential in the above state (b) will amount to about 1.5-1.6 volts with soft steel electrode relative to saturated calomel electrode (SCE) and in the alkaline medium.
Various and profound efforts have been made by those skilled in the art to reduce the hydrogen-generating voltage at the cathode as far as possible, as is commonly known.
The cathode is made generally of soft steel which is coated with nickel or a precious metal. In this conventional manner, however, the base electrode material or iron dissolves liably out into the aqueous caustic alkali solution especially at the hydrogen-generating potential balance, and in the form of HFeO.sub.2.sup.-. Therefore, it will naturally be encountered that, with increased effort to lower the overpotential in the above meaning, the iron dissolution will rather liably occur. Thus, it is a long desire among those skilled in the art to provide a cathodic electrode structure which satisfies the favorable overpotential requirement as well as an extended durable life of the cathodic electrode without sacrificing the electrolytic performance.
It has already been proposed to improve the cathodic electrode by replacing the iron with stainless steel or nickel as the base material. However, in these cases, disadvantageous dissolution will equally occur in the form of HFeO.sub.2.sup.- and HNiO.sub.2.sup.-, respectively, much difficulty being thereby encountered in keeping the precious metal surface layer in its stabilized and favorable operating conditions.
It is, therefore, an object of the present ivention to provide an efficient and durable cathodic electrode without the foregoing conventional drawbacks.